Poster L10 - Tissue-Engineered Small Artery Characterization via Fluid Dynamic Modeling

Saturday, October 26, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Roy Chen

Undergraduate Researcher
Duke University
Durham, North Carolina, United States

Co-Author(s)

Danielle Yarbrough

PhD Student
Johns Hopkins University
Durham, North Carolina, United States

Primary Investigator(s)

SG

Sharon Gerecht, PhD

Professor
Duke University, United States

Introduction: Vascular diseases are a leading cause of death globally but are difficult to study. In vitro models are essential for studying pathophysiology but must mimic in vivo conditions to be accurate. Key factors for vascular models include cellular components, mechanical properties, and flow conditions. Endothelial cells (ECs) form a monolayer in the lumen of vessels and regulate blood flow by responding to forces like shear stress. Smooth muscle cells (SMCs) around vessels regulate vascular tone based on EC signaling and circumferential strain due to changes in blood pressure. Mechanical properties such as compliance and elasticity affect how a vessel behaves when exposed to forces from blood flow and surrounding tissue. Flow conditions, characterized by flow rate and pulsatility, result in different stresses applied to the vessel. Arteries, specifically, experience high pressure and pulsatile flow, requiring extensive cellular activity to regulate rapidly changing conditions.
Existing vascular models have elucidated the importance of these mechanical factors on EC and SMC behavior. However, integrating all these factors remains a challenge due to the complexity of material engineering of the scaffold, tissue engineering techniques to cellularize the graft, and computational methods to validate fluid dynamics. To address this gap, we present an in vitro arterial model cultured under pulsatile flow, consisting of a fibrin scaffold surrounded by smooth muscle tissue. We computationally evaluate the material properties of the fibrin scaffolds and the applied flow regimes to ensure the cellularized vessels experience biologically relevant flow rates, wall shear stresses (WSS), and circumferential strains.

Materials and

Methods: Vessel scaffolds were made from electrospun fibrin. Smooth muscle cells were derived from human induced pluripotent stem cells and validated for phenotypic marker expression. Smooth muscle cells were cultured for two to three weeks before being wrapped around scaffolds as cell sheets. The resulting cellularized grafts were cultured with luminal flow for one week. Pulsatile flow was applied using a peristaltic pump. Distention was calculated from outer diameter measurements of the vascular grafts using a Keyence Laser Micrometer. Immunohistochemical imaging was used to confirm the presence of a continuous SMC layer. Ring tensile tests were conducted on the grafts to determine their elastic moduli.
Simulations were generated using the open-source program SimVascular (May 2023 release). The fluid structure interaction (FSI) between the fluid domain and the vessel wall is simulated using the coupled momentum method (CMM). The vessel wall was modeled with a uniform elastic modulus and uniform thickness along the length. The fluid solver uses the generalized minimal residual method (GMRES). Inflow boundary conditions were derived from in-line pressure measurements before and after the graft, using a lumped parameter model and assuming Poiseuille flow conditions to approximate flow rates. Outflow boundary conditions were prescribed based on a lumped parameter model of the flow setup.

Results, Conclusions, and Discussions: Results
Fluid dynamic simulations estimated WSS and distention of vascular grafts based on peristaltic pump flow conditions (Figure 1). Simulations showed that WSS is not significantly affected by changes in wall stiffness (Figure 1A). However, wall distention does significantly change with varying wall stiffness (Figure 1B).
For experimental testing, thin and thick vascular grafts were constructed and cultured in a custom bioreactor system (Figure 2A). Distention of the grafts was measured while varying flow rate (Figure 2B-D). The measured distention generally aligned with simulation results (Figure 1C). The cellularized grafts are also phenotypically similar to human arteries (Figure 2E-F).

Discussion
This work presents a robust in vitro platform for studying arterial behavior. We show that accurate computational modeling of flow regimes is crucial for ensuring physiological conditions are applied in the model when in situ measurements would be challenging. Simulation results confirm that WSS is primarily a function of flow rate and negligibly affected by wall stiffness. Maintaining biologically relevant WSS and pulsatility are essential because endothelial cells are highly sensitive to these flow conditions and can develop a diseased phenotype when WSS is low or oscillatory. However, material properties of the vessel wall are important to ensure the desired distention at a given flow rate, affecting the strain on SMCs in culture. Computational modeling, therefore, is vital for estimating the correct flow rate and elastic modulus of the graft needed to apply physiological conditions to the vascular graft.
This platform can be used directly to study various vascular diseases and for drug screening. Each component of the model, i.e., cell source, material properties of the scaffold, flow rate, flow media composition, etc., can be modified to study different pathologies. We demonstrated that computational tools could effectively calculate the physical forces on the graft based on flow rate and elasticity of the vascular grafts and that a cellularized vascular graft with SMCs can be constructed and validated. Thus, our platform enables precise control and customization of the in vitro environment to mimic specific disease conditions and advance the study of vascular disease mechanisms, toward discovery therapeutic target and drug screening.

Acknowledgements (Optional):